Some wireless communication devices, such as access points (APs) and/or mobile stations (STAB), may employ multiple-input and multiple-output (MIMO) technology to improve data throughput, to improve channel conditions, and/or to increase range. In general, MIMO may refer to the use of multiple antennas in a wireless device to achieve antenna diversity. Antenna diversity may allow the wireless device to choose to transmit and/or receive signals from a set of multiple paths through the wireless channel, which in turn may reduce the impact of multipath interference and increase channel diversity, for example, to provide well-conditioned wireless channels.
Antenna diversity may be achieved by providing polarization diversity, pattern diversity, and/or spatial diversity. Polarization diversity may be achieved by using multiple antennas with different polarizations to transmit or receive radio frequency (RF) signals. For example, a horizontally polarized antenna may be used to transmit and receive horizontally polarized signals, and a vertically polarized antenna may be used to transmit and receive vertically polarized signals. It is noted that a horizontally polarized antenna may not harvest sufficient energy from vertically polarized signals to successfully receive the vertically polarized signals, and a vertically polarized antenna may not harvest sufficient energy from horizontally polarized signals to successfully receive the horizontally polarized signals.
Pattern diversity may be achieved by using multiple antennas, each having a unique radiation pattern and/or radiation direction, to transmit or receive RF signals. More specifically, to achieve omni-directional signal transmission and reception coverage, multiple antennas may be positioned in different (e.g., orthogonal) directions so that their corresponding radiation patterns are oriented in different directions. For example, a horizontally polarized dipole antenna and a vertically polarized dipole antenna may be arranged in a “cross” configuration to provide an omni-directional radiation pattern. However, because the horizontally polarized dipole antenna may not be able to receive vertically polarized signals and the vertically polarized dipole antenna may not be able to receive horizontally polarized signals, cross-dipole antennas may not provide omni-directional signal coverage on the horizon (e.g., in the azimuth plane) for both horizontally polarized signals and vertically polarized signals. As a result, cross dipole antennas may not be suitable for use in WLAN applications (e.g., in access points and mobile stations) for which omni-directional signal coverage in the horizontal plane is desired for different polarization angles. In addition, positioning antennas in orthogonal directions may increase the space occupied by such antenna systems.
Spatial diversity may be achieved by spacing the multiple antennas apart from one another. Due to the small size and form factor of many wireless devices (e.g., APs and STAB), spatial diversity may be difficult to achieve in such wireless devices.
Thus, there is a need for a compact antenna structure that provides omni-directional coverage in the azimuth plane for signals of various polarizations.